Chemical composition and properties of ashes from combustion plants using Miscanthus as fuel

Miscanthus giganteus is one of the energy crops considered to show potential for a substantial contribution to sustainable energy production. In the literature there is little data available about the chemical composition of ashes from the combustion of Miscanthus and practically no data about their physical properties. However, for handling, treatment and utilization of the ashes this information is important. In this study ashes from two biomass combustion plants using Miscanthus as fuel were investigated. The density of the ashes was 2230 ± 35 kg/m~3, which was similar to the density of ashes from straw combustion. Also the bulk densities were close to those reported for straw ashes. The flowability of the ashes was a little worse than the flowability of ashes from wood combustion. The measured heavy metal concentrations were below the usual limits for utilization of the ashes as soil conditioner. The concentrations in the bottom ash were similar to those reported for ash from forest residue combustion plants. In comparison with cyclone fly ashes from forest residue combustion the measured heavy metal concentrations in the cyclone fly ash were considerably lower. Cl-, S and Zn were enriched in the cyclone fly ash which is also known for ashes from wood combustion. In comparison with literature data obtained from Miscanthus plant material the concentrations of K, Cl-and S were lower.This can be attributed to the fact that the finest fly ash is not collected by the cyclone de-dusting system of the Miscanthus combustion plants.     

Influence of temporally variable groundwater flow conditions on point measurements and contaminant mass flux estimations

Monitoring of contaminant concentrations, e.g., for the estimation of mass discharge or contaminant degradation rates, often is based on point measurements at observation wells. In addition to the problem, that point measurements may not be spatially representative, a further complication may arise due to the temporal dynamics of groundwater flow, which may cause a concentration measurement to be not temporally representative. This paper presents results from a numerical modeling study focusing on temporal variations of the groundwater flow direction. “Measurements” are obtained from point information representing observation wells installed along control planes using different well frequencies and configurations. Results of the scenario simulations show that temporally variable flow conditions can lead to significant temporal fluctuations of the concentration and thus are a substantial source of uncertainty for point measurements. Temporal variation of point concentration measurements may be as high as the average concentration determined, especially near the plume fringe, even when assuming a homogeneous distribution of the hydraulic conductivity. If a heterogeneous hydraulic conductivity field is present, the concentration variability due to a fluctuating groundwater flow direction varies significantly within the control plane and between the different realizations. Determination of contaminant mass fluxes is also influenced by the temporal variability of the concentration measurement, especially for large spacings of the observation wells. Passive dosimeter sampling is found to be appropriate for evaluating the stationarity of contaminant plumes as well as for estimating average concentrations over time when the plume has fully developed. Representative sampling has to be performed over several periods of groundwater flow fluctuation. For the determination of mass fluxes at heterogeneous sites, however, local fluxes, which may vary considerably along a control plane, have to be accounted for. Here, dosimeter sampling in combination with time integrated local water flux measurements can improve mass flux estimates under dynamic flow conditions.     

Model-based prediction of long-term leaching of contaminants from secondary materials in road constructions and noise protection dams

In this study, contaminant leaching from three different secondary materials (demolition waste, municipal solid waste incineration ash, and blast furnace slag) to groundwater is assessed by numerical modeling. Reactive transport simulations for a noise protection dam and a road dam (a typical German autobahn), in which secondary materials are reused as base layers, were performed to predict the breakthrough of a conservative tracer (i.e., a salt) and sorbing contaminants (e.g., PAHs like naphthalene and phenanthrene or heavy metals) at the groundwater table. The dam constructions have a composite architecture with soil covers in inclined layers and distinct contrasts in the unsaturated hydraulic properties of the used materials. Capillary barrier effects result in strong spatial variabilities of flow and transport velocities. Contaminant breakthrough curves at the groundwater table show significant tailing due to slow sorption kinetics and a wide distribution of travel times. While conservative tracer breakthrough depends primarily on subsoil hydraulic properties, equilibrium distribution coefficients and sorption kinetics represent additional controlling factors for contaminant spreading. Hence, the three secondary materials show pronounced differences in the temporal development of leached contaminant concentrations with consequences for breakthrough times and peak concentrations at the groundwater table. Significant concentration reductions due to dispersion occur only if the source concentrations decrease significantly prior to the arrival of the contaminant at the groundwater table. Biodegradation causes significant reduction of breakthrough concentrations only if flow velocities are low.     

Anaerobic degradation of vinyl chloride in aquifer microcosms

The anaerobic degradation potential at a chloroethene-contaminated site was investigated by operating two anoxic column aquifer microcosms enriched in iron(III). One column was fed with vinyl chloride (VC) only (column A) and one with VC and acetate (column B). In column A, after about 600 pore volume exchanges (PVEs), VC started to disappear and reached almost zero VC recovery in the effluent after 1000 PVEs. No formation of ethene was observed. In column B, effluent VC was almost always only a fraction of influent VC. Formation of ethene was observed after 800 PVEs and started to become an important degradation product after 1550 PVEs. However, ethene was never observed in stoichiometric amounts compared with disappeared VC. The average stable isotope enrichment factor for VC disappearance in column A was determined to be -4.3‰. In column B, the isotope enrichment factor shifted from -10.7 to -18.5‰ concurrent with an increase in ethene production. Batch microcosms inoculated with column material showed similar isotope enrichment factors as the column microcosms. These results indicated that two degradation processes occurred, one in column A and two in parallel in column B with increasing importance of reductive dechlorination with time. This study suggests that in addition to reductive dechlorination, other degradation processes such as anaerobic oxidation should be taken into account when evaluating natural attenuation of VC and that isotope analysis can help to differentiate between different pathways of VC removal.     

Biodegradation of Slow-Release Fertilizers (Methyleneureas) in Soil

The biodegradation of urea and condensation products thereof (ureaforms or methyleneureas), their nitrification, and their influence on the respiratory rate of soil was studied over periods of up to 100 days. The total methyleneurea content of the soil was determined after its acidic extraction, using a convenient colorimetric assay, and an HPLC protocol was established to analyze for specific components of methyleneureas. Urea, unfractionated methyleneureas, and hot-water soluble methyleneureas were rapidly metabolized to ammonium, which accumulated to high concentrations and was consequently oxidized to nitrate; an accumulation of nitrite was observed during urea but not during methyleneurea degradation. Hot water-insoluble methyleneureas were degraded much more slowly, and ammonium formed from these compounds was oxidized to nitrate without being released in significant amounts. These results suggest that the use of methyleneureas of optimized composition with regard to their water solubility may help to resolve problems such as the toxicity of ammonia to plant growth as well as nitrogen loss by leaching of nitrate, denitrification and volatilization.     

Effects of wind-driven circulation on river intrusion in Lake Tegel: modeling study with projection on transport of pollutants

Prediction of mixing intensity of water masses in riverine Lake Tegel (Berlin, Germany) can be used to trace the fate of pollutants that enter the lake through several inflows. Because the contributions of each inflow have not yet been quantified and because the lake features complex bathymetry and numerous islands, a hydrodynamic circulation model with high spatial resolution and dynamic wind forcing is useful. We applied the two-dimensional version of the Princeton Ocean Model to separate the influences of wind and river discharge on the currents and mass transport in Lake Tegel. For model validation, we compared the simulation results with 1?year of electrical conductivity data, which was used as a conservative tracer to distinguish between water from the River Havel and water supplied by a smaller second inflow. Calculation of currents alone is insufficient to investigate water exchanges between rivers and lakes, especially when several islands create multiple pathways for river intrusion. Therefore, mass transport simulations are applied. Our calculations based on archetypical scenarios indicate that the proportion of (polluted) water from the River Havel in the main basin of Lake Tegel fluctuates with river discharge and wind, which either amplify or neutralize each other.